For advanced bipolar transistors, the effect of parasitic series resistance can significantly degrade AC switching performance, particularly in light of the increasingly higher current densities. Advanced transistor designs minimize the resistances by using self-aligned device structures and polysilicon emitter technology. The self-alignment of the emitter-base minimizes the base resistance, while the polysilicon emitter allows for high base doping without degrading the current gain.
It is often desirable to incorporate advanced bipolar and advanced CMOS technology on the same integrated circuit. A simple method of achieving a "Bi-CMOS" device is to use the polysilicon gate process to simultaneously fabricate the polysilicon emitter of the bipolar transistor.
For purposes of reducing the emitter resistance, it is desirable to place a contact directly over the polysilicon which overlies the emitter of the transistor. In cases of submicron emitters, however, the alignment tolerance becomes critical, and therefore, some means is necessary to ensure alignment between the polysilicon and the metal contacts.
Two previously developed bipolar transistors use a single poly structure in a Bi-CMOS device. In the first transistor structure, a polysilicon region having an enlarged upper portion is disposed directly above the base region. The enlarged polysilicon portion allows a contact to be made to the emitter, even if the dimension of the emitter are less than the contact. Unfortunately, the enlarged polysilicon portion increases the distance between the emitter and the extrinsic base region, thereby increasing the base resistance of the transistor. Furthermore, the enlarged polysilicon portion is separated from the intrinsic base by an oxide layer, thereby increasing the transistor emitter-base capacitance.
Another single-poly transistor structure uses a polysilicon emitter which overlaps a field oxide region adjacent to the base of the transistor. While this structure reduces the separation of the extrinsic base and the emitter, the size of the emitter is dependent upon the alignment of the polysilicon pattern.
Therefore, a need has arisen to provide a single polysilicon transistor having reduced emitter and base resistances, while ensuring reliable contact to the transistor.